Researchers give warblers tiny backpacks to learn more about bird migration

Squinting through binoculars at the branches of a black spruce, Stephanie Szarmach caught sight of a yellow-rumped warbler carrying a small red band on its leg and a Tic Tac-size device on its back. She had placed the identifying bands the year before as part of her doctoral work, and the new tag recorded the bird's entire yearly cycle. She played a recording of the species song to draw the male into a mist net, then lifted him free to confirm the backpack remained secure after months of flight. Szarmach and her adviser David Toews attached these multi-sensor geolocators to dozens of myrtle warblers breeding near Anchorage. The tags combine light sensors with barometric pressure readings to track movements more precisely than earlier light-only devices, which lose accuracy near equinoxes. When a tagged bird climbs during migration the pressure drops sharply, marking departure and arrival times at each stop. Between flights the steady pressure values match the elevation of the resting site, and researchers match the full pressure pattern against weather records to locate the bird. Previous observations had suggested northwestern myrtle warblers might winter on the California coast, an idea first noted by Richard McGregor in 1899. The new tags showed every tracked individual instead flew east across the Canadian boreal forest before turning south to the Gulf Coast. The round-trip distance reached 6,800 miles, longer than expected for a species often viewed as a short- or medium-distance migrant. The six birds whose fall routes were mapped all followed the same broad eastward-then-southward path. The study demonstrates how pressure data can resolve migration questions for small songbirds that cannot carry heavier GPS units. Szarmach recovered the tags after the birds returned to Alaska, yielding the first detailed records of this population's annual movements.